The invention concerns a method of nuclear magnetic resonance (NMR) with which a sample of magnetic spins having differing Larmor frequencies and located in an external magnetic field are subjected to a sequence of at least two radio frequency pulse sequences separated in time by tm, wherein the differences in the Larmor frequencies are given either by the type of nuclei or in consequence of inhomogeneities of the magnetic field used or through the effect of an additional magnetic field gradient and wherein the first radio frequency pulse sequence effects production of a periodic modulation of the z-magnetization, with respect to the distribution of Larmor frequencies, having a modulation separation 2.DELTA..omega. and the second radio frequency pulse sequence transfers this periodically modulated z-magnetization into transverse magnetization, leading to a signal.
A method of this kind is e.g. known in the art from DE 34 45 689 A1.
The invention concerns an imaging method to improve the signal intensity of so-called stimulated echoes. Embodiments of this type of signal creation known to date have, compared to directly refocussed spin echoes, the disadvantage of a reduction in amplitude of 50%. The method in accordance with the invention discloses a way in which this disadvantage can be avoided leading to the creation of stimulated echoes having, in the limiting case, an amplitude identical to that of the spin echo.
The mechanism for creation of an NMR-signal in the form of a stimulated echo was introduced in 1950 by Hahn (Hahn, E. L., Spin Echoes, Phys.Rev. 80:580-594 (1950)). This work considered stimulated echoes as a special case of spin echoes. Modern vernacular, however, refers to spin echoes as those signals produced by refocussing of transverse magnetization using a radio frequency pulse. This nomenclature is utilized below.
The literature discloses a series of measuring processes based on signal production using stimulated echoes. Stimulated echoes have the particular advantage that, in contrast to spin echoes, the magnetization is maintained as z-magnetization during a portion of the preparation interval during which it is not subject to the dephasing mechanisms active for transverse magnetization. In addition, the magnetization decays during the associated time interval with the longitudinal relaxation time T1 which, in particular for in vivo applications, is substantially longer than the transverse relaxation time relevant for spin echoes T2. However, this advantage is associated with the fact that stimulated echoes have a reduction in single amplitude of 50% relative to spin echoes. For this reason, stimulated echoes are only preferred relative to spin echoes if this disadvantage is compensated by the length of the excitation sequence in conjunction with the slower T1 signal decay. This is currently the case, in particular, for methods for diffusion weighted imaging in accordance with the principal of Stejskal-Tanner. In most other NMR methods, spin echoes are preferentially used, since the signal loss of approximately a factor of 2 is viewed as being disadvantageous due to the intrinsic signal to noise problems in NMR.
FIGS. 2a-2g show the mechanism for forming stimulated echoes according to prior art. The components Mx, My and Mz are thereby shown as a function of the resonance frequency (and thereby of the magnetic induction B.sub.0 at the location of the nuclei due to the Larmor relationship .omega.=.gamma.B.sub.0 with .gamma.=the geomagnetic ratio). The following description assumes that the magnetization is equally distributed as a function of .omega.. Such an equal distribution can be most easily effected through application of a constant magnetic field gradient across the sample under investigation, should it not already be present due to the type of sample under investigation.
In this case, the following conditions obtain: All magnetization is initially directed as z-magnetization having an amplitude M.sub.0. (FIG. 2a). Mz is transferred into Mx via a 90.degree. pulse having phase .gamma.. (FIG. 2b). Following a time te, the transverse magnetization diephases. Mx=M.sub.0, cos(.omega.te), My=M.sub.0 sin(.omega.te). (FIG. 2c). An additional 90.degree. pulse once more transfers Mx into Mz with My initially remaining constant. (FIG. 2d). Following an additional waiting time tm, the transverse magnetization My decays with the time constant T2 due to the transverse relaxation, with Mz remaining basically preserved due to the assumed substantially longer T1. (FIG. 2e). The sinusoidal modulated z-magnetization is once more transferred into Mx by means of an additional 90.degree. pulse. (FIG. 2f).
Following a time interval te, which is identical to the time interval te between steps b) and c), the transverse magnetization is modulated in accordance with Mx=M.sub.0 cos.sup.2 (wte), My=M.sub.0 cos(.omega.te)sin .omega.te. The observed signal amplitude S results from the integral over the transverse magnetization and is therefore S=1/2M.sub.0. This signal is the stimulated echo (FIG. 2g).
This illustration of the stimulated echo generation mechanism in accordance with FIGS. 2a-2g clearly shows that the signal loss of 50% is due to the fact that, when applying the second 90.degree. pulse, half of the entire magnetization (namely Mx) is transferred into z-magnetization, whereas the other half remains as transverse magnetization and therefore decays during the time interval tm.